After root canal treatment, the tooth is weak and brittle. It is weak because the center of the tooth was removed to get at the infected nerve; only the sides of the tooth are left for support. And it is more brittle because the nerve and blood supply are gone.
This tooth was not crowned after root canal treatment. There is a tremendous amount of force on the edges of the teeth when they bite together, so it's likely that this tooth will break.
After a tooth breaks, it is much more difficult to repair. Our Houston dentist, Dr. Minh Nguyen, typically recommends a crown to cover and protect a root canaled tooth. The crown helps to prevent it from breaking.